1. Field of the Invention
The present invention relates to a semiconductor device formed by using a semiconductor film, which includes a switching element. Also, the present invention relates to an OLED panel in which the switching element and an organic light emitting diode (OLED) formed on a substrate is enclosed between the substrate and a cover member. Also, the present invention relates to an OLED module in which an IC is mounted on the OLED panel. Note that, in this specification, the OLED panel and the OLED module are generically called light emitting devices.
2. Description of the Related Art
In recent years, technology for forming a thin film transistor (hereafter referred as TFT) on a substrate has been largely improved, and an application development of the TFT to an active matrix type display device has been carried out. In the active matrix type display device, a TFT as a switching element is provided in each pixel and video signals are sequentially written in each pixel to display an image. A TFT is an indispensable element in realizing the active matrix type display device.
The TFT also becomes a switching element, which has three terminals of a source electrode, a drain electrode, and a gate electrode. An electric resistance between the source electrode and the gate electrode is controlled by the voltage, which is applied to the gate electrode.
With the increasing needs for rapid increases in display quality and amount of display information, and for making displays lighter and thinner, the demand for higher definition pixels in active matrix display devices is rising. Further, higher functionality such as incorporating memory into pixels is also being sought after more and more.
However, even if pixels are given higher definition higher functionality, there is a limitation in how much the size of TFTs formed in each pixel can be shrunk when considering how to ensure the amount of on electric current, the resistance to voltage, and the like.
However, for cases in which the proportion of the pixel surface area occupied by the TFTs cannot be reduced, the surface area through which light passes becomes smaller in pixels of a liquid crystal display device, and the brightness as seen by eye becomes lower. Further, even in light emitting devices, light from an OLED is blocked by TFTs when the light emitted by the OLED is irradiated to the TFT side, and the brightness as seen by eye becomes lower.
It is therefore preferable to reduce the number of TFTs formed in each pixel, and the surface area of the TFTs, by as much as possible.
However, provided that the circuit structure of each pixel has been determined, it is normally not possible to simply reduce the number of TFTs. For example, it is necessary to short-circuit or to open three nodes A, B, and C at the same in each of the pixels as shown in FIG. 15A, and therefore at least two TFTs 3001 and 3002 must be formed if three-terminal switching element TFTs are used. The number of TFTs cannot be reduced any further.
In particular, the number of TFTs formed in each pixel is generally large, their connections are complex, and it is difficult to suppress the number of TFTs for the case of active matrix light emitting devices as compared with liquid crystal display devices in which there is no need for anything other than simple write-in switching of voltage signals.